Thrusters currently utilized for satellite or other space stationkeeping and maneuvering applications utilize propellant gases with relatively low exhaust velocities (in the range of approximately 500 meters/sec to 2000 meters/sec). Examples of such thrusters include cold gas thrusters, which typically utilize valved nitrogen as the propellant and have very low specific impulses, and hydrazine thrusters, which are the thrusters most commonly used, but which also provide low specific impulses (although four times that of the cold gas thrusters). Hyrdazine thrusters also present tankage problems related both to liquid handling in zero gravity and to storing an unstable and highly corrosive fuel. Other available thruster technologies, including Teflon ablative thrusters, Hall thrusters, ion thrusters and MPD thrusters, while offering higher specific impulses, suffer from a variety of other problems, including being relatively massive, lack of temporal agility and/or requiring significant electrical energy storage, all of which has prevented use of such devices for space stationkeeping and maneuvering applications.
Since the lift weight of a satellite or other space vehicle is normally predetermined, the more weight or mass required for thruster propellants, the less is available for payload. It is therefor desirable to keep propellant mass to a minimum. Thus, since the thrust which can be achieved from a given mass of propellant increases substantially linearly with exhaust velocity, if exhaust velocity can be increased by for example a factor of ten, then the mass of propellant can either be reduced by a factor of ten, or the same mass or quantity of fuel/propellant will last ten times longer, thereby potentially extending the useful life of the space vehicle.
Another problem faced in industry is that as the density of integrated circuits and other micro-products formed using lithographic techniques increases, the wavelength of the radiation used for lithographic etching needs to be correspondingly reduced. In particular, for the next generation of lithography, radiation in the extreme ultraviolet (EUV) band, which extends from approximately 10A.degree. (1 nm) to 1000A.degree. (100 nm), and in particular at a wavelength approximately 130A.degree. (13 nm) it is deemed critical. However, the only radiation source capable of operating in this band is large, cumbersome, expensive and operates at too low a pulse repetition frequency (PRF) for lithographic and many other applications. A practical source for generating radiation in this band, and in particular a source generating radiation at 13 nm, does not currently exist. A need therefor exists for a radiation source operating in this wavelength band which source is of usable size and cost and which generates radiation at wavelength and PRFs suitable for lithographic and other applications. More generally, a need exists for an EUV radiation source capable of generating radiation over at least a significant portion of this band, which source can be designed or programmed relatively easily and predictably by selecting various parameters to produce radiation at a desired wavelength within this band. In addition to lithography, such source might find application in various imaging or detection systems.
As is discussed later, plasma gun technology may be applied to dealing with the above problems. However, existing plasma guns have had reliability and pulse repetition frequency (PRF) limitations which has prevented their applicability in space applications, where long-term maintenance-free operation and high PRFs are requirements, and the relatively low PRFs has also prevented such plasma guns from being used for lithography. In particular, prior art coaxial plasma guns have required a very high power, extremely fast switch to instantaneously produce the drive. Large spark gap switches, which were the only components available which met the requisite specifications, have never operated at PRFs in excess of 100 Hz or for more than a few million shots without maintenance. As a result, plasma guns have never had PRFs exceeding 10 Hz. For space applications, PRFs in excess of 5000 Hz (pulses/sec) and maintenance free cycles exceeding 100 million pulses are desirable, while for lithography, PRFs of at least 500 Hz and preferably 1000 Hz are required.
Further, prior art plasma guns have utilized a dielectric insulator at the base of a coaxial column to create a voltage enhancement which helps force breakdown or plasma initiation at that point. Reliable, uniform plasma initiation could only be produced by applying a very high voltage very rapidly, and the dielectric is often quickly damaged by the resulting breakdown. Reliability and low PRF problems have therefor prevented plasma guns from being utilized as thrusters in space applications or as EUV radiation sources for lithographic or other applications. A need therefor exists for an improved plasma gun which provides the maintenance free reliability required for space applications along with relatively high PRFs, preferably in excess of 5,000 Hz for space, while being adapted to deliver exhaust velocities of 10,000 to 100,000 meters/sec. for space, and preferably in excess of 1000 Hz for radiation applications.